Apple - Can't start Xcode - stuck on installing state

There is a small progress bar under the Launchpad icon in the Menu Bar.

You can also see the current download progress by hovering over the icon.

Wait for it to end.

In the Updates tab of the Apple App Store, after waiting 1 hour for "2 minutes left," I hit CMD+R to refresh and that seemed to "jump start" it. It finished within 60s after that. Could have been coincidence though (YMMV).

In launch pad hold the Xcode icon until the X appears and click the X to delete Xcode then go back to the app store and get a fresh copy.